In industries, the use of metal products manufacturing by compaction and sintering metal powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced and the quality requirements are continuously raised at the same time as it is desired to reduce the cost. As net shape components, or near net shape components requiring a minimum of machining in order to reach finished shape, are obtained by pressing and sintering of iron powder compositions in combination with a high degree of material utilisation, this technique has a great advantage over conventional techniques for forming metal parts such as moulding or machining from bar stock or forgings.
One problem connected to the press and sintering method is, however, that the sintered component contains a certain amount of pores, decreasing the strength of the component. Basically there are two ways to overcome the negative effect on mechanical properties caused by the component porosity. 1) The strength of the sintered component may be increased by introducing alloying elements such as carbon, copper, nickel molybdenum etc. 2) The porosity of the sintered component may be reduced by increasing the compressibility of the powder composition, and/or increasing the compaction pressure for a higher green density, or increasing the shrinkage of the component during sintering. In practise, a combination of strengthening the component by addition of alloying elements and minimising the porosity is applied.
There are three common ways of alloying iron powders: prealloying, admixing and diffusion alloying. One advantage of prealloying is that a good distribution of the alloying elements is guaranteed throughout the alloy. However, the disadvantage is that compressibility is reduced with alloying element content in a prealloyed material. When adding alloying elements by admixing, compressibility is not affected adversely. However, distribution and segregation problems can occur, since the alloying element particles often need to be much smaller than the base material particles in order to promote diffusion during sintering. Diffusion bonding is a technique that offers the middle path solution. The alloying elements are admixed to the base material, followed by a heat treatment in a reducing atmosphere, thereby bonding the smaller alloying element particles by diffusion to the larger particles, decreasing the risk of segregation while upholding good compressibility.
Chromium as an alloying element serves to strengthen the matrix by solid solution hardening. Chromium will also increase hardenability, oxidation resistance and abrasion resistance of a sintered body. Solutions exist today including chromium as alloying element. However, these products require very well controlled atmospheres during sintering in order to generate positive effects. The present invention is directed towards an alloy excluding chromium, thus resulting in lower requirements on sintering furnace equipment and/or control.
During sintering, metal powder particles of the compacted or pressed component, the green component, will diffuse together in solid state forming strong bonds, so called sintering necks. The result is a relatively high dense net shape part, or near net shape part, suitable for low or medium performance applications. Typically, sintered articles are manufactured from iron powder mixed with copper and graphite. Other types of materials suggested include iron powder prealloyed with nickel and molybdenum and small amounts of manganese to enhance iron hardenability without developing stable oxides. Machinability enhancing agents such as MnS are also commonly added.
Various automotive parts have been produced successfully by the pressing and sintering technique. It is desirable to improve the performance of sintered parts so that more parts can be replaced by this cost effective technique. However, automotive parts manufacture is a high volume and price sensitive application with strict performance, design and durability requirements. Therefore cost-efficient materials are highly desirable.
U.S. Pat. No. 3,901,661, U.S. Pat. No. 4,069,044, U.S. Pat. No. 4,266,974, U.S. Pat. No. 5,605,559, U.S. Pat. No. 6,348,080 and WO 03/106079 describe molybdenum containing powders. When powder prealloyed with molybdenum is used to produce pressed and sintered parts, bainite is easily formed in the sintered part. In particular, when using powders having low contents of molybdenum, the formed bainite is coarse impairing machinability, which can be problematic in particular for components where good machinability is desirable. In addition, molybdenum is a very expensive as alloying element.
However, in U.S. Pat. No. 5,605,559 a microstructure of fine pearlite has been obtained with a Mo-alloyed powder by keeping Mn very low. It is stated that Mo improves the strength of steel by solution hardening and precipitation hardening of Mo carbide, and the like. However, when Mo content is less than about 0.1 wt-%, its effect is small. Mn improves the strength of a heat-treated material by improving its hardenability. However, when Mn content exceeds about 0.08 wt-%, oxide is produced on the surface of alloyed steel powders such that compressibility is lowered. However, keeping the Mn content low can be expensive, in particular when using cheap steel scrap in the production, since steel scrap often contains Mn of 0.1 wt-% and above. Thus, a powder produced accordingly will be comparably expensive.
U.S. Pat. No. 4,954,171 describes a powder to be used for the production of sintered parts by powder metallurgy and a high-strength sintered alloy steel. However, said alloy contains high amounts of Mo, as 0.65-3.50 wt-% is claimed. The results presented have been obtained by using costly processing routes, such as double compaction and high temperature sintering.